Niskoo is a great app for teaching English spelling and vocabulary. It is Ad free and has a lot of free content.

Phonics
What sound do different letter combinations make? This gets easier with a focus on phonetics instead of individual letters. Now you can learn the hidden pronunciation rules of English in a fun way!

Montessori
Part of Montessori learning is student led learning. With this app you are in control and can choose whatever scene and word you want to learn. The scenes allow you to learn words in context.

Content Led
Learn the words that you are interested in! Choose different scenes and understand how words fit into the bigger picture. You get to choose the word to spell instead of getting random words.

Spelling and Vocabulary modes
Want to skip the spelling and just know what something is in the picture? Change to vocabulary mode and touch on anything to know what it is.

Intrinsic Motivation
Real learning comes from personal achievement. There is no reward system, only the sweet knowledge that you are constantly learning and getting better. Way to go!